Kinetic Trapping of Pressure-Induced Symmetry Breaking in Hybrid Metal Halides via Fluorine-Mediated Molecular...

Permanently capturing pressure-induced metastable phases at ambient conditions is a fundamental challenge that, if solved, would unlock a new realm of materials with exotic functionalities. A central obstacle is the reversibility of pressure-driven transitions, particularly symmetry-breaking ones that yield valuable properties like nonlinear optical activity. Here, we report a kinetic trapping strategy that permanently preserves high-pressure noncentrosymmetric phases in hybrid tin bromides via a fluorine-mediated "molecular lock." Using a two-dimensional perovskite (3-CF₃py)₂SnBr₄ (3-CF₃py⁺ = 3-(trifluoromethyl)pyridinium) as a model system, we demonstrate that hydrostatic compression induces a symmetry-breaking transition at ∼4.0 GPa with a strong second-harmonic generation (SHG) response, which peaks at ∼8 GPa with a staggering ∼400-fold enhancement relative to the signal at ∼4.0 GPa. Remarkably, the noncentrosymmetric phase and its SHG activity remain after full pressure release. Single-crystal x-ray diffraction reveals that pressure-enhanced directional F···F interactions cooperatively reorganize the organic-inorganic framework, suppressing elastic recovery and locking in the metastable structure. Importantly, this design principle is generalizable, as demonstrated by trapping of the high-pressure noncentrosymmetric phase in the one-dimensional analogue, (3-CF₃py)SnBr₃. Our work establishes a powerful design strategy to induce and stabilize high-pressure phases, making their otherwise inaccessible functionalities available for ambient-condition applications.